1. Field of the Invention
The present invention relates to a bottom resist layer composition that serves as an antireflective coating composition and used for micro processing in manufacturing processes of semiconductor devices and so on. The present invention particularly relates to a bottom resist layer composition of a multilayer-resist film suitable for exposure using far ultraviolet ray, KrF excimer laser light (248 nm), ArF excimer laser light (193 nm), F2 laser light (157 nm), Kr2 laser light (146 nm), Ar2 laser light (126 nm), soft x-ray (EUV, 13.5 nm), electron beam (EB), or the like. Furthermore, the present invention relates to a process for patterning a substrate by lithography using the bottom resist layer composition.
2. Description of the Related Art
As packing density and speed of LSIs have become higher in recent years, a finer pattern rule has been demanded. In lithography using optical exposure which is used as a general technique at present, resolution is reaching the inherent limit defined by a wavelength of a light source.
Optical exposure has been widely used using g-line (436 nm) or i-line (365 nm) of a mercury-vapor lamp as a light source for lithography when a resist pattern is formed. It has been considered that a method of using an exposure light with a shorter wavelength is effective as a means for achieving a further finer pattern. For this reason, KrF excimer laser with a shorter wavelength of 248 nm has been used as an exposure light source instead of i-line (365 nm), for mass-production process of a 64 M bit DRAM processing method. However, a light source with far shorter wavelength is needed for manufacture of DRAM with a packing density of 1 G or more which needs a still finer processing technique (a processing dimension of 0.13 μm or less), and lithography using ArF excimer laser (193 nm) has been particularly examined.
On the other hand, it has been known so far that a bilayer-resist process is excellent for forming a pattern with a high aspect ratio on a nonplanar substrate. Furthermore, in order to develop a bilayer resist film with common alkaline developers, a high-molecular silicone compound having a hydrophilic group such as a hydroxy group or a carboxyl group is necessary as a base resin of a top resist layer composition.
As a silicone chemically amplified positive-resist composition, proposed is a silicone chemically amplified positive-resist composition for KrF excimer laser in which polyhydroxy benzyl silsesquioxane, which is a stable alkali-soluble silicone polymer, in which some phenolic hydroxyl groups are protected by t-Boc groups is used as a base resin, and the base resin is combined with an acid generator (see Japanese Patent Application Laid-open (KOKAI) No. 06-118651 and SPIE vol. 1925 (1993) p377). Moreover, as the silicone resist composition for ArF excimer laser, proposed is a silsesquioxane-based positive resist composition in which cyclohexyl carboxylic acids are substituted with acid labile groups (see Japanese Patent Application Laid-open (KOKAI) No. 10-324748; Japanese Patent Application Laid-open (KOKAI) No. 11-302382; and SPIE vol. 3333 (1998) p62). Furthermore, as the silicone resist composition for F2 laser, proposed is a silsesquioxane-based positive resist composition having hexafluoro isopropanols as soluble groups (see Japanese Patent Application Laid-open (KOKAI) No. 2002-55456). The above-mentioned polymers include in their main chains poly silsesquioxane containing a ladder structure formed by condensation polymerization of a trialkoxy silane or a tri halogenated silane.
As a resist base polymer having silicon-pendant side chains, silicon-containing (meth)acrylate polymers are proposed (see Japanese Patent Application Laid-open (KOKAI) No. 09-110938 and J. Photopolymer Sci. and Technol. Vol. 9 No. 3 (1996) p435-446).
A bottom layer used for a bilayer-resist process is required a hydrocarbon compound which can be etched with changed.
It follows from FIG. 3 that a sufficient antireflection effect to reduce reflectivity of a substrate to 1% or less is obtained by making an intermediate resist layer to have a low k value of 0.2 or less and a proper thickness.
In general, an antireflective coating with a thickness of 100 nm or less is required to have a k value of 0.2 or more in order to reduce the reflectivity of a substrate to 1% or less (see FIG. 2). However, an intermediate resist layer of a trilayer structure has an optimum k value of 0.2 or less because a bottom resist layer can reduce reflection to some extent.
Next, FIG. 4 and FIG. 5 show fluctuations of reflectivity of a substrate while the thickness of an intermediate resist layer and the thickness of a bottom resist layer are changed in the case that a bottom resist layer has a k value of 0.2 or 0.6.
The bottom resist layer having a k value of 0.2 in FIG. 4 is intended to represent a bottom resist layer optimized for the bilayer process. The bottom resist layer having a k value of 0.6 in FIG. 5 has the k value similar to those of novolac or polyhydroxy styrene at a wavelength of 193 nm.
The thickness of a bottom resist layer fluctuates depending on topography of a substrate. On the other hand, the thickness of an intermediate resist layer is oxygen gas. The bottom layer is required to have high etching resistance because the layer functions as a mask in the case of etching its underlying substrate. When the bottom layer is etched with oxygen gas, the layer is required to consist of only hydrocarbons without silicon atoms. Moreover, in order to improve a line width controllability of a silicon-containing top resist layer and to reduce irregularities on pattern sidewalls and pattern collapse caused by standing waves, the bottom resist layer is also required to function as an antireflective coating, specifically to reduce reflectivity from the bottom resist layer to the top resist layer down to 1% or less.
Then results of calculating fluctuations of reflectivity of a substrate while a thickness of a bottom resist layer is changed in the range of 0 to 500 nm are shown in FIG. 1 and FIG. 2. It is premised that an exposure wavelength is 193 nm, n value of a top resist layer is 1.74 and k value thereof is 0.02. FIG. 1 shows fluctuations of reflectivity of a substrate while k value of a bottom resist layer is fixed at 0.3, n value is changed in the range of 1.0 to 2.0 in the ordinate axis, and a thickness of a bottom resist layer is changed in the range of 0 to 500 nm in the abscissa axis. In the case of assuming a bottom resist layer with a thickness of 300 nm or more for a bilayer resist process, optimum values that realize reflectivity of 1% or less exist in the range of 1.6 to 1.9 which is as much as or a little higher than that of a top resist layer.
FIG. 2 shows fluctuations of reflectivity of a substrate while n value of a bottom resist layer is fixed at 1.5, and k value thereof is changed in the range of 0 to 0.8. Reflectivity of 1% or less is realized when k value is in the range of 0.24 to 0.15. Meanwhile, an optimum k value of an antireflective coating used with a thin thickness of about 40 nm for a monolayer resist process is 0.4 to 0.5, which is different from an optimum k value of a bottom resist layer used with a thickness of 300 nm or more for a bilayer resist process. Therefore, it is shown that a bottom resist layer for a bilayer resist process is required to have lower k value, namely, higher transparency.
Then as a bottom resist layer composition for a wavelength of 193 nm, a copolymer of a polyhydroxy styrene and an acrylate has been examined as disclosed in SPIE Vol. 4345 (2001) p50. Polyhydroxy styrene has an extremely strong absorption at a wavelength of 193 nm. Polyhydroxy styrene alone has a high k value of about 0.6. Then the k value is adjusted to about 0.25 by carrying out copolymerization with an acrylate whose k value is almost 0.
However, acrylates exhibit lower etching resistance than polyhydoroxystyrene during etching of a substrate. Moreover, in order to lower the k value, acrylates have to be copolymerized so that the acrylates account for considerable ratio. As a result, etching resistance during etching of a substrate is significantly lowered. The etching resistance influences not only an etch rate but also generation of surface roughness after etching. The copolymerization of acrylates cause serious increase of surface roughness after etching.
One of moieties that are more transparent at a wavelength of 193 nm and higher etching resistance than a benzene ring is a naphthalene ring. For example, Japanese Patent Application Laid-open (KOKAI) No. 2002-14474 discloses a bottom resist layer comprising a naphthalene ring or an anthracene ring. However, a naphthol copolycondensation novolac resin and a polyvinyl naphthalene resin have k values in the range of 0.3 to 0.4. These k values do not reach the target transparency of 0.1 to 0.3, and thus transparency of the resins have to be increased. In addition, the naphthol copolycondensation novolac resin and the polyvinyl naphthalene resin have low n values at a wavelength of 193 nm. According to measurement results of the present inventors, the naphthol copolycondensation novolac resin has an n value of 1.4, and the polyvinyl naphthalene resin has such a low n value of 1.2. Acenaphthylene polymers disclosed in Japanese Patent Application Laid-open (KOKAI) No. 2001-40293 and Japanese Patent Application Laid-open (KOKAI) No. 2002-214777 also have lower n values at a wavelength of 193 nm than those at a wavelength of 248 nm, high k values, and thus neither n value nor k value reaches the target value.
Meanwhile, a trilayer resist process is proposed. In the trilayer resist process, there are stacked a single layer resist without silicon as a top resist layer; under the top resist layer, an intermediate resist layer containing silicon atoms; and under the intermediate resist layer, an organic layer as a bottom resist layer (for example, see J. Vac. Sci. Technol., 16(6), November/December 1979).
In general, a single layer resist is superior in resolution to a silicon-containing resist. Therefore, a single layer resist exhibiting high resolution may be used as an exposure imaging layer in the trilayer resist process.
As the intermediate resist layer, Spin On Glass (SOG) films are used. Many SOG films have been proposed.
An optimum optical constant of a bottom layer to reduce reflection from a substrate in a trilayer resist process is different from that in a bilayer resist process.
The purpose to suppress reflection from a substrate as much as possible, specifically, to suppress a reflectivity of a substrate to 1% or less is the same in both a bilayer process and a trilayer process. However, antireflection effect is given only to a bottom layer in the bilayer process, while the antireflection effect may be given to either an intermediate layer or a bottom layer, or to both of them in the trilayer process.
U.S. Pat. No. 6,506,497 specification and U.S. Pat. No. 6,420,088 specification disclose a silicon-containing layer composition having antireflection effect.
In general, multi-layer antireflective coatings have higher antireflection effect than single layer antireflective coatings. Therefore, the multi-layer antireflective coatings have been widely and industrially used as antireflective coatings for optical materials.
High antireflection effect may be obtained by imparting antireflection effect to both an intermediate resist layer and a bottom resist layer.
When a silicon-containing intermediate resist layer functions as an antireflective coating in the trilayer process, a bottom resist layer is not particularly required to have a superlative function as an antireflective coating.
A bottom resist layer in the trilayer process is required to have high etching resistance during processing of a substrate rather than effects as an antireflective coating.
Therefore, novolac resins have been used as bottom resist layers for the trilayer process because novolac resins have high etching resistance and large amounts of aromatic groups.
Next, FIG. 3 shows fluctuations of reflectivity of a substrate while k value of an intermediate resist layer is wavelengths, excellent etching resistance under conditions of etching substrates, and is promising for forming a bottom resist layer used for a multilayer-resist process such as a silicon-containing bilayer resist process or a trilayer resist process using a silicon-containing intermediate layer.
In order to achieve the above mentioned object, the present invention provides a bottom resist layer composition for a multilayer-resist film used in lithography comprising, at least, a polymer comprising a repeating unit represented by the following general formula (1),

wherein R1 represents a hydrogen atom or an acid labile group;
X represents any one of a single bond, —Y—C(═O)—, and a linear or branched alkylene group having 1-4 carbon atoms;
Y represents a single bond or a linear or branched alkylene group having 1-4 carbon atoms;
Z represents any one of a methylene group, an oxygen atom, and a sulfur atom;
R2 and R3 independently represent an alkyl group, an considered to hardly fluctuate and that the intermediate layer can be applied with a prescribed thickness.
It follows from FIGS. 4 and 5 that the reflectivity of a substrate can be reduced to 1% or less with a thinner thickness in the case of a bottom resist layer with a higher k value (k=0.6).
In the case of a bottom resist layer with a k value of 0.2 and with a thickness of 250 nm, an intermediate resist layer is required to be thick in order to achieve the reflectivity of 1%.
Increase of the thickness of an intermediate resist layer results in large load to a topmost resist layer during dry etching for processing the intermediate resist layer, and which is not preferable.
In recent years, finer patterns have been rapidly realized. In the dimension of 45 nm LS, resists having a thickness less than 100 nm are used in view of preventing pattern collapse. Also in the trilayer process, it has become difficult to transfer resist patterns having a thickness equal to or less than 100 nm to a silicon-containing intermediate resist layer. Therefore, thinner silicon-containing intermediate resist layers have increasingly used. It follows from FIGS. 4 and 5 that use of a silicon-containing intermediate resist layer having an absorption of a k value about 0.1 achieves the reflectivity of 1% or less independent of k value of a bottom resist layer, for example, when the silicon-0≦d≦0.9, 0≦e≦0.9, and 0<b+c+d+e<1.0.
As mentioned above, the bottom resist layer composition comprising a polymer comprising a repeating unit represented by the general formula (1) exhibits optimum n value and k value on exposure to short wavelengths such as 193 nm, and excellent etching resistance under conditions of etching substrates. Therefore, such a bottom resist layer composition is useful for forming a bottom resist layer used for a multilayer-resist process such as a silicon-containing bilayer resist process or a trilayer resist process using a silicon-containing intermediate resist layer.
Then a bottom resist layer formed with the bottom resist layer composition can be more transparent than layers formed with polyhydroxystyrene, cresol novolac, naphthol novolac, or the like. In addition, the bottom resist layer with a thickness of 200 nm or more exhibits excellent antireflection effects on exposure at short wavelengths such as 193 nm.
“Waviness” of a bottom resist layer pattern after a substrate is etched has been pointed out. A phenomenon is reported that hydrogen atoms of a bottom resist layer are substituted with fluorine atoms during etching of a substrate with a fluorocarbon gas (Proc. of Symp. Dry. Process, (2005) p11). It is considered that finer pattern waviness is caused because the surface of a bottom resist layer is turned into Teflon (registered trade mark) and containing intermediate resist layer has a thickness of 50 nm. However, there is a demand of using silicon-containing intermediate resist layers having a thickness equal to or less than 50 nm in view of enhancing accuracy of etching the silicon-containing intermediate resist layers. Silicon-containing intermediate resist layers with a thickness of 50 nm or less exhibit antireflection effects about half of the intermediate resist layers with a thickness more than 50 nm. Therefore, bottom resist layers underlaying such intermediate resist layers are required to have n values and k values similar to those of the bottom resist layers for a bilayer resist process.
Against this backdrop, there has been expected to develop a composition for forming a bottom resist layer that exhibits optimum n value and k value on exposure to shorter wavelengths, excellent etching resistance under conditions of etching substrates, and is promising for forming a bottom resist layer used for a multilayer-resist process such as a silicon-containing bilayer resist process or a trilayer resist process using a silicon-containing intermediate resist layer.